Patent GB-1477475-A (GB)
Title: Reproducing colour television recordings
IPC: H04N9/873
Filed: 1974-07-12
Abstract: 1477475 Colour television PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 12 July 1974 [17 July 1973] 30972/74 Heading H4F The frames of a PAL colour television signal having a chrominance modulated carrier in a frequency band. below that of a carrier frequency-modulated by the luminance information are read from a record carrier in a sequence different from that in which they were recorded to reproduce slow-motion or still pictures, the read-out signal being supplied to a terminal S, Fig. 1, the luminance-modulated carrier being separated at 3 and demodulated at 15; the chrominance modulated carrier at frequency fc is separated at 4 and supplied to mixing stages 13, 21 which receive respective reference signals at frequencies fs + fc and fs - fc where fs is a desired (standard) subcarrier frequency, band-pass filters 14, 22 separating from the mixer stage outputs chrominance modulated carriers each of frequency fs and of complementary phases which are fed to a switch 25a; switch 25a is controlled by a device 26 which also controls the sequence in which the frames are read-out so that the chrominance modulated carrer at the output of switch 25a alternates in phase as required by the PAL system. Thus for still reproduction switch 25a is operated after each frame period so that the carrier on line 1 of each frame has a complementary phase to the carrier on line 625 of the previous frame. The record carrier may be magnetic or an optically coded disc having one frame recorded per revolution, movement of the scanning device at right-angles to the recorded track by an odd number of tracks (e.g. one for still reproduction) causing operation of switch 25a. The reference signals at frequencies fs + fc and fs - fc are derived by band-pass filters 12, 20 receiving the output of a mixing stage 11 which receives inputs from oscillators 10 and 6 operating at frequencies fs and fc respectively, the frequency of oscillator 6 being locked to the frequency of the line synchronizing pulses separated at 9 from the luminance signal output. Signal dropouts may be detected by a circuit 18 which controls switches 19, 19a and 24. Detection of a signal dropout causes switch 19 to pass to luminance output Y the output of a demodulator 17 receiving the output of a one line delay 16, switch 19a establishing a closed loop including delay 16. Signal dropout detection also causes switch 24 to pass to chrominance output F the output of a one-line delay 23, a switch 25b ganged with switch 25a, ensuring that the output of delay 23 has the same phase as the output of switch 25a. To provide compensation in the chrominance signal for dropouts of more than one line period the second input of switch 24 may be connected to the output of a further switch, operated, during dropout, at the end of each line period, and having two inputs connected to the outputs of respective delays connected to the outputs of switches 25a, 25b, which delays form parts of respective closed loops, as with delay 16, during dropout. In an alternative embodiment, Fig. 2, in which switches 24 and 15 have been interchanged relative to Fig. 1, detection of a signal dropout establishes a closed loop including one line delays 23a, 23b.